1. Field of the Invention
The present invention relates to a technique for stabilizing voltages and currents to be outputted from a power supply circuit of a self-oscillation and resonance type (hereinafter, referred to as a self-oscillation-resonance type power supply circuit) obtained by improving the Royer oscillation circuit.
2. Description of the Related Art
Power-supply circuits for supplying driving voltages to a load include those adopting various types of circuits and driving methods. As power-supply circuits used to respond to a load requiring an alternating-current (AC) driving voltage, self-oscillation-resonance type power supply circuits are known. It has been obtained by improvement of the Royer oscillation circuit. The self-oscillation-resonance type power supply circuit has a push-pull configuration including a self-oscillation driving method to output voltages involving a resonant phenomenon.
The self-oscillation-resonance type power supply circuit as described above has advantages in that its configuration is simpler and it easily outputs substantially a sine-wave AC voltage. Because of these advantages, the circuit (which is now reduced in size) is being widely used as a small-capacitance inverter circuit. Furthermore, because of the advantageous feature that the circuit outputs substantially a sine-wave voltage, the circuit is also us ed as a DC-to-DC converter circuit for an apparatus which request to generate seldom noise.
In the past, there was substantially no case in which self-oscillation-resonance type power supply circuits had thereto controllers that regulate output voltages so as to be constant. This is because since the self-oscillation-resonance type power supply circuit performs self-oscillation operations in coincidence with the occurrence of the resonant phenomenon, application of a control means, such as a PWM (pulse-width modulation) controller, thereto was considered to be difficult. Therefore, as shown in FIG. 1, a power-supply unit has been constructed with an inverter formed of a self-oscillation-resonance type power supply circuit and a converter that possesses an output-voltage controller, in which voltages to be fed to a load are stabilized by control of voltages to be fed from the converter to the inverter.
Such a circuit including both the inverter and the converter, however, requires more components and a larger substrate space for configuration of the converter than in the case of an inverter-based circuit. This produces problems in that the total scale of the power-supply unit is increased, and in addition, production cost is increased.